Guy Webster 818-354-6278 ΚΚΚ
Jet Propulsion Laboratory, Pasadena, Calif.
December 17, 2002
News Release: 2002-231
Galileo Millennium Mission Status
NASA's Galileo spacecraft has begun transmitting high-priority scientific
information that was collected and stored on its tape recorder during the orbiter's
early-November dash by Jupiter, which brought it closer to the planet than ever
before.
Damage from naturally strong radiation near Jupiter had left the tape recorder
inoperable for weeks. Galileo's flight team traced the problem to a light-emitting
diode in the electronics controlling the motor drive, and then gradually and
carefully completed a successful long-distance repair job.
"We're delighted playback has begun. There was no guarantee we could get to this
point," said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion
Laboratory, Pasadena, Calif. "It's a real tribute to the dedication and creativity of
the team, and it continues the tradition set by the larger Galileo teams earlier in the
mission who overcame challenging setbacks to keep the mission a success."
Playback and transmission began late last week, after tape movements earlier in
the week positioned the tape to start the download with the data of greatest scientific
interest. That information is from a period starting on Nov. 4, when Galileo was
beginning to get closer to Jupiter than the moon Io's orbit. It ends when the
spacecraft shut down its instruments on Nov. 5 as part of a precautionary standby
reaction to other radiation-induced problems with the spacecraft's computer
shortly after passing the inner moon Amalthea.
"We hope this will be the best data set that's ever been collected about the inner
region of Jupiter's magnetic environment," said JPL's Dr. Torrence Johnson,
Galileo project scientist. The tape playback may also provide recorded information
about dust particles that form Jupiter's faint "gossamer" ring, which Galileo flew
through on Nov. 5.
The recovery was achieved by running a current through the damaged diode to
anneal, or repair, radiation-caused damage. The first annealing attempt of six
hours produced barely discernible improvement. Three additional treatments, for a
total of 83 more hours of annealing treatment, produced progressive
improvements, to the point that the tape recorder can run for about an hour at a
time. A fifth treatment produced no additional gain. However, normal playback runs
the tape recorder for only a few minutes at a time, so the improvement appears
sufficient, said JPL engineer Greg Levanas, who helped plot the recovery strategy.
The flight team plans to continue playback until mid-January, the scheduled end of
Galileo's mission operations.
Galileo's tape recorder became critical to the mission's strategy for handling
science data after the spacecraft's main antenna failed to open fully during the
journey from launch in 1989 to arrival at Jupiter in 1995. Most information
from onboard science instruments has been recorded onto tape during the busy days
when the spacecraft has flown near one of Jupiter's moons. There have been over
30 such flybys since 1995. The recorded data has been played back slowly for
transmission to Earth via Galileo's smaller secondary antenna during the weeks and
months before each subsequent flyby.
Galileo has operated five years longer than its original prime mission and has
received more than four times the cumulative dose of harmful radiation it was
designed to withstand. Last month's encounter took the orbiter nearly twice as close
to Jupiter as it had ever been before, and exposed it to the strongest radiation it has
yet experienced. In particular, the spacecraft entered the most intense region of a
trapped proton belt and was exposed to 40 times the proton radiation of any single
previous passage close to Jupiter, probably more than the rest of the mission
combined.
The diode that radiation apparently damaged in the tape recorder is a
gallium-arsenide semiconductor component that emits light. The motor-drive
control has three of them. Light from them shines through windows in a rotating
wheel onto detectors on the other side of the wheel. That setup senses the turning of
the wheel and feeds digital logic that controls drive signals for the motor.
The damage apparently came from high-energy protons from Jupiter's radiation
belt displacing atoms in the semiconductor's crystalline molecular lattice. Passing a
current through the diode for hours serves as a way for electron flow to cause some
of the displaced atoms to shift back to their original lattice positions.
Galileo has nearly depleted its supply of the propellant needed for pointing its
antenna toward Earth and controlling its flight path. While still controllable, it has
been put on a course for impact into Jupiter next September. The maneuver
prevents the risk of Galileo drifting to an unwanted impact with the moon Europa,
where it has discovered evidence of a subsurface ocean that is of interest as a
possible habitat for extraterrestrial life.
Additional information about Galileo and the discoveries is available at
http://galileo.jpl.nasa.gov .
JPL, a division of the California Institute of Technology in Pasadena, manages the
Galileo mission for NASA's Office of Space Science, Washington, D.C.
-end-